NetXMS Support Forum

Seems that you do have a DNS records NETXMS_SERVER or it's in your hosts file. Web console uses it as one of the options for situations when web console in installed on another machine. See here:
https://www.netxms.org/documentation/adminguide/installation.html?highlight=netxms_server#installing-web-interface-on-remote-system

I definitely do not have an entry anywhere but by adding it I can now log in to the WebUI.  So does it not attempt local before checking for NETXMS_SERVER or should there always be an entry for NETXMS_SERVER?

It tries several ways, as per documentation, to get server address and takes localhost as last resort. Normally none of these is set. Could be that something changed after you launched the web api on that server.

Thanks so much for the help and guidance, it's much appreciated.  All working happily now and my frustration is over